1. Technical Field
This invention relates generally to thermal print media, and more particularly to minimizing the curl induced into transparent media during thermal printing.
2. Background Art
As is well known in the art, a thermal printhead utilizes a row of closely spaced electrically-resistive heater elements which are selectively energized to record data in hard copy form. To produce a high quality continuous-tone print using the thermal printing process, a dye donor media (sheet or web) and dye receiver media (sheet or web) are passed through the nip formed between a thermal printhead and a platen while heat is selectively applied to the dye donor sheet by image-wise energizing the heater elements that make up the thermal printhead. Dye is transferred from the dye donor media to the dye receiver media, which includes a dye receiver layer coated on an opaque or transparent support for reflective prints or for transparencies, respectively. A transparent support may be comprised of any suitable light transmissive material such as polyester film.
Projection transparency dye receiver media exhibits an objectionable amount of curl (also referred to as warpage) as a result of heating the surface of the dye receiver media nearest to the thermal head during dye transfer. The curl problem becomes severe as attempts are made to print transparencies with high optical image densities (D.sub.max of about 1.8 or greater) which require the support surface nearest to the print head to experience temperatures significantly higher than the glass transition temperature (T.sub.G) of most common transparent support materials employed in transparencies. The curl of a projection transparency affects the quality of projected images, and it is preferred to have the finished projection transparency as flat as possible when placed upon a flat surface.
Prior art approaches to controlling curl include the use of synthetic paper dye receiver media of at least three plies, each having different Cobb sizing degrees or internal bond strength to prevent curling when used for facsimile, thermal printing, etc. Another prior art approach involves the use of a paper support containing pigment and a rubbery polymer latex material providing a material that would yield reduced curl when imaged with a thermal head or heat pin. Still another curl prevention technique involves coating the surface of the substrate with a layer of resin that is neither heat expandable or contractible; preferably an acrylic resin. These approaches, however, are unsuitable for transparencies because they increase opacity and/or add additional expense to the cost of manufacturing the media due to the addition of materials and/or production steps.
U.S. Pat. No. 4,892,994, which issued to Masaru Tsuchiya et al. on Jan. 9, 1990, discloses a guide passage at the outlet of a thermal transfer step. The guide passages bend in the direction opposite to the induced curl to reduce the effect of induced curl in the finished print. It would be desirable to have a method for producing transparencies that prevent the formation of curl, rather than apparatus that reduces curl once formed.
Commonly assigned U.S. Pat. No. 5,220,351, which issued to Martin et al on Jun. 15, 1993, discloses a method for minimizing curl of transparent media during printing in a thermal printer, comprises wrapping the transparent media about a circumference of a printer platen so that the transparent media contacts the circumference for an arc angle of between about 60 degrees to about 230 degrees. While this is effective in reducing curl, it does require a re-design of the media transport path, and is therefore not suitable for existing printers. It would be advantageous to provide a method for minimizing curl of transparent media during thermal printing with existing printers.